The small GTPase Rac controls cell morphology, gene expression, and reactive oxygen species formation. Manipulations of Rac activity levels in the cerebellum result in motor coordination defects, but activators of Rac in the cerebellum are unknown. P-Rex family guanine-nucleotide exchange factors activate Rac. We show here that, whereas P-Rex1 expression within the brain is widespread, P-Rex2 is specifically expressed in the Purkinje neurons of the cerebellum. We have generated P-Rex2 ^−/− and P-Rex1 ^−/− /P-Rex2 ^−/− mice, analyzed their Purkinje cell morphology, and assessed their motor functions in behavior tests. The main dendrite is thinned in Purkinje cells of P-Rex2 ^−/− pups and dendrite structure appears disordered in Purkinje cells of adult P-Rex2 ^−/− and P-Rex1 ^−/− /P-Rex2 ^−/− mice. P-Rex2 ^−/− mice show a mild motor coordination defect that progressively worsens with age and is more pronounced in females than in males. P-Rex1 ^−/− /P-Rex2 ^−/− mice are ataxic, with reduced basic motor activity and abnormal posture and gait, as well as impaired motor coordination even at a young age. We conclude that P-Rex1 and P-Rex2 are important regulators of Purkinje cell morphology and cerebellar function.